1. Field of the Invention
The present invention relates to a friction clutch having at least a pair of frictional power transmission surfaces such as an electromagnetic clutch suitably used for driving an automotive air-conditioning system.
2. Description of the Related Art
As described in Japanese Unexamined Patent Publication No. 8-74885, for example, in an electromagnetic clutch often-used, conventionally, for driving an automotive air-conditioning system, an armature, which is supported elastically and movably, is moved axially by the magnetic attraction force generated in a magnetic coil and pressed against a rotor thereby to generate a friction force between the armature and the rotor and thus to transmit a torque. This structure, however, requires a large pressure to be applied to the armature and hence a greater amount of current supplied thereto, thereby posing the problem of a bulky clutch with an increased cost.
In order to obviate the aforementioned problem of the prior art, the object of the invention is to provide a friction clutch or, specifically, an electromagnetic clutch having a new configuration in which a strong pressure is generated on the friction surfaces for transmitting a larger torque while reducing the size of the whole clutch by reducing the required magnetic force and thus reducing the size of the coil and the rotor providing a magnetic path.
According to the invention, as a means for solving the problem described above, there is provided a friction clutch comprising a rotor having a first friction surface supported rotatably around a driven shaft and rotated by an external power source, a boss mounted on the driven shaft, an intermediate member having a second friction surface adapted to engage, in contact, with the friction surface of the rotor and pivotally supported on the boss for converting the friction force generated with the friction surface of the rotor into the pressure against the rotor, and initial operating means for applying the initial force to the intermediate member for pressing the friction surface of the intermediate member against the friction surface of the rotor.
In this friction clutch, the rotational power is transmitted by engagement in contact between the friction surface formed on a drive-side rotor rotatably supported on the driven shaft or a portion adapted to be rotated integrally with the rotor and the friction surface formed on the intermediate member pivotally supported on the boss mounted on the driven shaft. When the friction surface of the intermediate member is pressed against the friction surface of the rotor by a comparatively small force generated in the initial operating means, the friction force acting between the friction surfaces rotates the intermediate member around the pivotal shaft, so that the friction force due to the initial torque is converted into a large pressure exceeding the pressure generated by the initial operating means, the friction surface of the intermediate member is strongly pressed against the friction surface of the rotor, and a strong frictional engaging force is generated between the two friction surfaces, thereby making it possible to transmit a large rotational power. In this way, a large frictional engaging force is obtained in spite of a small pressure generated in the initial operating means and, therefore, the initial operating means can be reduced in size, with the result that the whole clutch can be reduced in size and cost.
According to another aspect of the invention, as another means for solving the problem described above, there is provided a friction clutch comprising a rotor supported rotatably around a driven shaft and rotationally driven by an external power, a boss having a first friction surface and mounted on the driven shaft, an intermediate member having a second friction surface adapted for contact engagement with the friction surface of the boss and pivotally supported on the rotor for converting the friction force generated with the friction surface of the boss into a pressure against the boss, and initial operating means for applying to the intermediate member the initial power for pressing the friction surface of the intermediate member against the friction surface of the boss.
In this aspect of the friction clutch, the rotational driving power is transmitted by contact engagement between the friction surface formed on the boss mounted on the driven shaft and the friction surface formed on the intermediate member pivotally supported on the drive-side rotor. In this case, when the friction surface of the intermediate member is pressed against the friction surface of the rotor with a comparatively small force generated in the initial operating means, the friction force acting between these friction surfaces rotates the intermediate member around the pivotal shaft, so that the friction force due to the initial torque is converted into a large pressure exceeding the pressure generated by the initial operating means, the friction surface of the intermediate member is strongly pressed against the friction surface of the boss, and a strong frictional engaging force is generated between these friction surfaces, thereby making it possible to transmit a large rotational power. In this way, a large frictional engaging force is obtained in spite of a small pressure generated in the initial operating means, and therefore the initial operating means can be reduced in size with the result that the whole clutch can be reduced in size and cost.
As described above, in order to generate, between the two friction surfaces in sliding contact a large pressure exceeding the pressure generated in the initial operating means, a mechanism for moving the intermediate member is desirably adapted to move the second friction surface of the intermediate member at least in the direction along the normal to the first friction surface. For this purpose, the second friction surface of the intermediate member is arcuate and the point at which the intermediate member is rotatably and pivotally supported is displaced from the driven shaft providing the rotational center of the second friction surface, whereby the direction of movement of the intermediate member has a component in the direction along the normal to the first friction surface.
In the above cases, the friction clutch according to the invention can be an electromagnetic clutch employing a configuration in which an electromagnetic coil is used as the initial operating means and the armature of a magnetic material formed on the intermediate member is attracted by the magnetic force. As a specific result of forming the friction clutch according to the invention with an electromagnetic clutch, the requirement of a smaller magnetic force makes it possible to reduce the size of the rotor constituting a magnetic path by reducing the number of turns of the electromagnetic coil or by reducing the wire diameter and thereby to reduce the size of the whole clutch. With the friction clutch according to the invention, the rotor is formed in such a manner that the first friction surface is normal to the driven shaft or forms a cylindrical surface coaxial with the driven shaft, while the second friction surface is formed in the intermediate member in an opposed relation to the first friction surface. The electromagnetic coil may be formed as an immovable member fixedly on the housing or movably on the rotor in a rotatable manner. In the latter case, a sliding contact mechanism including a brush and a slip ring is provided to supply power to the electromagnetic coil.
In the case where the friction clutch according to the invention comprises a plurality of intermediate members, the intermediate members are mounted in such a manner that the angle between each pair of adjoining intermediate members around the driven shaft is uniform thereby to offset the forces acting in radial direction and thereby to reduce the burden on the bearing. Also, in the case where a plurality of intermediate members are involved, an elastic member is desirably mounted between an end of one intermediate member and an end of another intermediate member in order to urge the friction surface formed on the intermediate members away from the friction surface formed on the rotor. Further, a balancer is desirably mounted on each of the intermediate members so that the center of gravity of the intermediate members coincides with the rotational center thereof and thereby to prevent the intermediate members from rotating around the pivotal shaft as the rotational center due to the centrifugal force.
In the case where an electromagnetic clutch is used as a friction clutch according to this invention, the friction surface for transmitting the torque and the magnetic pole surface for forming a magnetic path (or the friction surface) can be provided not as a common part but as separate members at least on the intermediate members and/or the rotor. As a result, the friction surface and the magnetic pole surface can be formed of a material suitable for the respective members. Thus, a friction surface with a small wear and free of seizure and a magnetic pole surface having a small magnetic reactance and a strong magnetism can be formed without affecting each other, thereby making it possible to produce a large transmission ability and a highly efficient initial operation characteristic.
A rubber-like elastic member can be attached around the pin shaft for supporting the intermediate member rotatably and slightly movably along the axial direction of the driven shaft. This can prevent the fretting wear liable to develop on the surface of the pin and the surface of the hole for receiving the pin shaft.